Benzo (rst) pentaphene, a potent six-membered carcinogen, and three of its fluoro derivatives, although similar in size and configuration, differ from each other in their ability to induce tumors and to act as tumor inhibitors. A study of their physicochemical properties relevant to carcinogenesis and their enzymatic activation by liver microsomal hydroxylases, chemical and physical binding to DNA and their effect as inducers of these enzymes will be undertaken. This is a case study in chemical carcinogenesis in view of the recently proposed theories, which will bring more experimental data for the evaluation of these theories.